Mechanism and Energetics for Dehydrogenation of Methane by Gaseous Iridium Ions

Ir[sup +] has been observed as the most efficient transition-metal ion for dehydrogenation of CH[sub 4] in the gas phase. We carried out theoretical studies of the complete reaction profile for this process and find three salient factors responsible for the high reactivity: (i) the ability of Ir[sup +] to change spin easily, (ii) the strength of the Ir-C and Ir-H bonds, and (iii) the ability of Ir[sup +] to form up to four covalent bonds. We show that among transition-metal ions Ir[sup +] is unique in best possessing all these characteristics. The combination of these factors leads to a global minimum for the singlet Ir(H)[sub 2](CH[sub 2])[sup +] structure, which plays an important role in the activation. On the basis of these results, we suggest solution-phase analogues that may also activate CH[sub 4]. 40 refs., 3 figs., 1 tab.